DE4128687A1 - FIBER OPTICAL SENSOR - Google Patents

Description

Technical field

The invention relates to the field of optical Measurement of electrical quantities. It affects a fiber top table sensor, comprehensive

• a) a light source;
• b) a piezoelectric sensor element;
• c) a first two-mode fiber with an input end and an output end, in which the LP ₀₁ basic mode and the straight LP ₁₁ mode can spread, and which is at least partially fixed to the sensor element so that a dimensional change of the Sensor element in an electrical field leads to a change in length in the fiber; and
• d) means for measuring the field-related change in length the fiber;

Such a fiber optic sensor is e.g. B. from EP-A1- 04 33 824 known.

State of the art

In various publications such. B. the European Patent applications EP-A1-03 16 619 and EP-A1-03 16 635 or the articles by H. Bohnert and J. Nehring in Appl. Opt. 27, pp. 4814-4818 (1988), or Opt. Lett. 14,  Pp. 290-292 (1989), are already fiber optic sensors Measurement of electrical fields and voltages described been used.

The measuring principle used is based on the inverse Piezo effect in materials with selected crystal sym metry. The temporally periodic dimensional change, the a corresponding piezoelectric body in an elec experiences alternating field, is on one on the body fixed fiber transmitted. The change in length of the company ser is then proportional to the field or voltage ampli tude and is measured and evaluated interferometrically.

Different types of glass fiber interferometers can be used for interferometric measurements. Because of its simplicity, one of these types is that of the article by BY Kim et al., Opt. Lett. 12, pp. 729-731 (1987), known two-mode fiber interferometer of particular interest. The parameters of the sensor fiber are selected in this interferometer so that exactly two modes (the LP ₀₁ basic mode and the straight LP ₁₁ mode) can propagate in the fiber.

The two-mode fiber interferometer turns light into one coherent light source, e.g. B. a laser diode, by a Two-mode fiber sent, which on a piezoelectric fixed sensor element for the electric field E. is. The two modes are stimulated by the light and spread differently in the fiber. On the fiber At the end you can observe an interference pattern, which ches from the overlay of these two fashions gives. A change in the length of the fiber leads to a differential phase shift between both modes, resulting in a corresponding change in the interfe renzpatters expresses.  

The two adjacent substructures of the interference pattern are detected with two detectors (e.g. in the form of photodiodes). Two signals V ₁₁ and V ₁₂ are phase-shifted at their output:

V₁₁ = (1/2) V₀ (1 + a * cosΦ (t)) (1)

V₁₂ = (1/2) V₀ (1-a * cosΦ (t)) (2)

with Φ (t) = A * sinΩt + R (t). The phase shift Φ (t) between the two modes is thus composed of a temporally periodic component A * sinΩt (A is proportional to the amplitude of the field) caused by the alternating field to be measured and an arbitrary phase term R (t), which is e.g. B. can also change over time due to temperature-related fluctuations in fiber length. Finally, V ₀ is proportional to the optical power and a is a measure of the interference contrast.

The searched term A * sinΩt is often obtained with a homodyne detection method from the output signals of the detectors (for a fiber-optic sensor with single-mode fiber see: DA Jackson et al., Appl. Opt. 19, p. 2926 -2929 (1980); a corresponding fiber-optic sensor with two-mode fiber is described in the European application EP-A1-04 33 824 cited at the beginning). In this method, the sensor fiber is additionally passed through a piezoelectric modulator. With the help of this modulator 4 , the phase difference R (t) is regulated to + (pi / 2) or - (pi / 2) (modulo 2pi). The modulator is part of a control circuit consisting of the detectors, a subtractor and a quadrature controller, which the differential voltage

V = V₁₁ - V₁₂ = V₀ * a * cosR (t) (3)

regulates accordingly to zero.

The two components A * sinΩt and R (t) of the phase shift are thus just compensated for by the modulator via a corresponding (opposite) change in length of the fiber. The voltage applied to the modulator then contains a slowly varying component which is proportional to R (t) and a periodic component which is proportional to A * sinΩt. The searched portion A * sinΩt is filtered out via a high-pass filter and can be taken from the signal output. The output signal is therefore independent of any fluctuations in the laser intensity (ie V ₀ ) and the interference contrast a.

In a number of practical applications of the sensor (e.g. when measuring voltage in outdoor systems) relatively large distances between the real Chen sensor head and the sensor electronics occur (10 m up to some 100 m). It is inappropriate to use these gaps to bridge itself with the two-mode fiber because the influence of external disturbances (temperature fluctuations, mechanical shocks etc.) with increasing fiber length increased accordingly and the signal / noise Ver ratio worsened. The light supply from the laser diode to the interferometer and the feedback of the off Rather, the interferometer's output signals should be about separate glass fibers are made that are not part of the Are interferometers.

In the homodyne method described above with a active phase modulator would be in addition to the ver binding glass fibers also an electrical connection between the sensor electronics and the sensor head control of the modulator required. The attractiveness  of a sensor working with this type of interferometer would be very limited.

It is therefore in two older German patent applications conditions (file numbers P 41 14 253.5 and P 41 15 370.7) beat, instead of the known active signal de tection, which has an additional modulator in the measuring fiber with appropriate electrical supply line required, to provide passive signal detection based on the Guoy effect (see: S. Y. Huang et al., Springer Proc. in Physics, Vol. 44 "Optical Fiber Sensors", pp. 38-43, Springer Verlag Berlin, Heidelberg (1989)), d. H. the Phase difference between the interference patterns of the Near and far field, is based: The substructures of the near and far field (4 in total) are in the sensor head separated with optical means and can be separated Glass fibers to a remote evaluation electronics will wear. There you can use at least three of these four substructures the desired information can be obtained from the change in length of the measuring fiber.

With this proposed solution, one becomes full constant electrical separation between sensor head and Evaluation electronics reached. However, this advantage bought in that additional optical components (Beam splitter) and a relatively complex electronics must be used. In addition, a mono mode laser diode used, the special measures to suppress light backscatter from the Sen sor required in the diode.

Presentation of the invention

It is therefore an object of the present invention a fa to create a seroptic sensor, on the one hand a  fault-insensitive separation of sensor head and evaluation electronics makes it possible, but also through simple construction and high measuring accuracy.

The task is for a sensor of the type mentioned Art solved in that

• e) the light source is a multi-mode laser diode;
• f) the measuring means a second, similar to the first Include two-mode fiber;
• g) the parameters of the two two-mode fibers so ge are chosen that the interference contrast for the Path differences of both modes in the individual two Moden fibers and for the sum of the path differences is approximately zero each time; and
• h) between the light source and the input end of the first two-mode fiber, and the output end of the first two-mode fiber and the input end of the second two-mode fiber each a single-mode fiber are provided for transmitting the light.

The essence of the invention is based on using the principle of white light interferometry for the measurement: The light of a multi-mode laser diode is fed to the first two-mode fiber via the insensitive first single-mode fiber. The coupling takes place in such a way that the two modes which can be expanded (LP ₀₁ basic mode and straight LP ₁₁ mode) are excited, preferably with the same intensity. In the first two-mode fiber, the modes experience an optical path difference Delta L ₁ for which the interference contrast is negligibly small. Therefore, no interference effects occur at the end of the sensor fiber. The two modes are coupled in roughly equal parts into a second insensitive monomode fiber and transmitted to a detection unit. There, the light is coupled again into a second two-mode fiber, which is identical in terms of its fiber parameters and length to the first two-mode fiber, in particular is identical in particular.

The coupling in turn takes place in such a way that both modes are excited to approximately the same extent. Light that has spread in the first two-mode fiber in the basic mode LP ₀₁ spreads in approximately equal parts in the LP ₀₁ - and in the straight LP ₁₁ mode of the second two-mode fiber. The same applies to light that has spread in the first two-mode fiber in the LP ₁₁ mode.

At the output end of the second two-mode fiber, there are path differences of zero, delta L ₁ and 2 * delta L ₁ between the light waves of the LP ₀₁ mode on the one hand and the waves of the LP ₁₁ mode on the other hand. Mode components with the gait difference zero then interfere with each other, while the remaining components provide a constant intensity background.

At this point it should be noted that such system called a tandem interferometer already for conventional interferometers (Michelson) is known from the prior art (see, for example, A. S. Gerges et al., Appl. Optics Vol. 29, No. 30, pp. 4473-4480 (1990)).

A preferred embodiment of the invention is characterized is characterized in that

• a) at least partially the second two-mode fiber a piezoelectric modulator is fixed;
• b) two at the output end of the second two-mode fiber Detectors for measuring the intensities of the two Fashions are arranged;
• c) the output signals of the two detectors via one Subtractor on the input of a quadrature controller reach;  
• d) the output of the quadrature controller control the modulator ert; and
• e) the output signal of the quadrature controller via a High pass is given to a signal output.

Such a known homodyne detection with active working point control and compensation of the AC Measurement signal has the advantage of a high sensitivity speed with great stability.

Further embodiments result from the dependent ones Claims.

Brief description of the drawing

The invention is intended below with reference to exemplary embodiments play explained in connection with the drawing will. Show it

Figure 1 shows the basic scheme of a tandem interfe rometer with two-mode fibers according to the inven tion.

Fig. 2 shows the interference contrast for a V with a multi-mode laser diode Michelson interferometer illuminated in dependence on the path difference Delta L;

Fig. 3A, the intensity I at the output of a tandem Inter ferometers for the interference package 0th Ord voltage in dependence on the difference in the transition difference (Delta L ₁ - Delta L _2);

FIG. 3B shows the representation corresponding to FIG. 3A for "interference packets" with orders up to ± 2;

Fig. 4, the intensity I for V = 1 as a function of the phase Φ; and

Fig. 5 shows the schematic structure of a preferred embodiment of the sensor according to the invention with homodyne detection.

Ways of Carrying Out the Invention

The basic (schematic) construction of a sensor according to the invention and the propagation of a wavefront of an individual, arbitrarily picked waves are shown in FIG. 1. The light from a light source 1 , a multi-mode laser diode, first spreads out as basic mode LP _{01 of} a first (polarization-maintaining) single-mode fiber 2 a. The observed wavefront stimulates two new wavefronts in a first two-mode fiber 4 a connected to the first monomode fiber 2 a by a first splice 3 a, which propagate in LP ₀₁ - and in straight LP ₁₁ mode. Both fronts are still in phase at the beginning of the fiber because they are generated by a common starting front. The coupling to the first double-mode fiber in the first splice 3 a is preferably carried out such that the intensities of the Inten LP ₀₁ - and LP ₁₁ modes approximately equal.

Since the effective refractive indices n (LP ₀₁ ) and n (LP ₁₁ ) are different for the two modes, there is an optical path difference between the two fronts at the end of the first two-mode fiber 4 a

Delta L = Delta L₁ = l (n (LP₀₁) -n (LP₁₁)) (4)

accumulated (1 here denotes the length of the first two-mode fiber 4 a). Delta L is such that at the end of the most two-mode fiber 4 a there are no interference effects. The two wave fronts stimulate two new fronts in a second monomode fiber 2 b (also polarization-maintaining) coupled via a second splice 3 b that both spread in LP ₀₁ mode. The Gangun difference Delta L is therefore preserved in this fiber. The coupling takes place in such a way that the LP ₀₁ and LP ₁₁ modes are coupled into the second monomode fiber 2 b in approximately equal proportions.

In a second, coupled via a third splice th two-mode fiber 4 b, the two wave fronts coming from the second single-mode fiber 2 b each excite two new fronts, one of which is located in the LP ₀₁ - and one in the LP ₁₁ Fashion spreads. The LP ₀₁ and LP ₁₁ modes in turn have approximately the same intensities (in order to be able to better track the wavefronts, those in FIG. 1 are those that originate from the LP ₀₁ mode of the first two-mode fiber 4 a, with a dash, and those that come from the LP ₁₁ mode with two dashes).

The path difference Delta L between the LP ₀₁ and LP ₁₁ front pairs is maintained along the second two-mode fiber 4 b. However, a phase difference Delta L ₂ accumulates between the LP ₀₁ front pair on the one hand and the LP ₁₁ front pair on the other. If the two-mode fibers 4 a and 4 b have the same fiber parameters and length, the following applies: | Delta L ₂ | = | Delta L ₁ | = | Delta L |. At the output end of the second two-mode fiber there are path differences of zero, delta L ₁ and 2 * delta L ₁ between the light waves of the LP ₀₁ mode on the one hand and the waves of the LP ₁₁ mode on the other hand. Mo components with a zero path difference then interfere with each other, while the other components provide a constant intensity background. The interference is finally evaluated in a subsequent detection unit 5 .

To further understand the tandem two-mode fiber interferometer according to the invention, a single Michelson interferometer with a multi-mode laser diode is first considered as a light source, as described in the already cited article by AS Gerges et al. is described. Applying the interference contrast V as a function of the optical path length difference Delta L between the two interferometer branches, there is a series of equidistant peaks of increasing order ± 1, ± 2, ± 3, etc., which are symmetrical with respect to the peak at Delta L = 0 lie ( Fig. 2). The interference contrast V is defined as

where I _max and I _{min are} the intensities of the maxima and minima of the interference fringe pattern. The location of the peaks is given by

Delta L = 2 * p * l _cav p = 0, ± 1, ± 2, (6)

with the atomic number p and the optical length l _{cav of} the laser resonator.

V between the peaks is very small, so there is almost no interference here. For equal amplitudes of the interfering waves, V (delta L = 0) = 1. With increasing order | p | the height H of the tips drops monotonically, according to the relationship K (delta L) = exp (-Delta L / L _cm ), where L _{cm is} the coherence length corresponding to the spectral laser mode width. So K goes for | Delta L | »L _cm towards zero.

In the two-mode fiber interferometer, the branches of the Michelson interferometer are replaced by the LP ₀₁ and LP ₁₁ modes. With increasing fiber length l, the path difference Delta L = l (n (LP ₀₁ ) -n (LP ₁₁ )) between the no den increases linearly with l. The interference contrast V (l) as a function of the fiber length then runs through the same series of peaks as in the Michelson interferometer. The maxima are at fiber lengths

l _p = 2 * p * l _cav / (n (LP₀₁) -n (LP₁₁)). (7)

The fiber length difference between neighboring peaks is consequently

Delta l = 2 * l _cav / Delta n _eff (8)

with delta n _eff = n (LP₀₁) -n (LP₁₁).

So that at the end of the first two-mode fiber 4 a no interference effects occur, either the path length dependent on the fiber length delta L must be much larger than L _cm , or l must be in a minimum between two peaks of the function V (l ) (interference effects at the end of the first two-mode fiber 4 a would result in undesirable fluctuations in intensity).

The light intensity at the output of the tandem two-mode fiber Interferometer is given by (analogous to equation (7) in the article by A. S. Gerges et al.):

I = I₀ {1 + V (Delta L₁) cos (Φ₁) + V (Delta L₂) cos (Φ₂)
+ (1/2) V (Delta L₁ + Delta L₂) cos (Φ₁ + Φ₂)
+ (1/2) V (Delta L₁-Delta L₂) cos (Φ₁-Φ₂)} (9)

Φ₁ and Φ₂ are the delta L₁ and Delta L₂ corresponding phase differences:

f ₀ is the average frequency of the laser diode, c is the speed of light. I _o is proportional to the optical power of the laser diode. Delta L ₁ and Delta L ₂ are given by

Delta L₁ = l₁ * Delta n _eff ⁽¹⁾ (12)

Delta L₂ = l₂ * Delta n _eff ⁽²⁾ . (13)

l₁ and l₂ are the lengths of the two two-mode fibers 4 a and 4 b; Delta n _eff ⁽¹⁾ and Delta n _eff ⁽²⁾ are the effective refractive index differences of the modes.

As already mentioned, delta n _eff ⁽¹⁾ = delta n _eff ⁽²⁾ = delta n _eff , l₁ = l₂ = l and consequently delta L₁ = delta L₂ = delta L. For a given fiber type, l is then chosen such that

V (Delta L₁) = V (Delta L₂) = V (Delta L) ≈ 0 (14)

is, and that also

V (Delta L₁ + Delta L₂) = V (2 * Delta L) ≈ 0 (15)

is.

Equation (9) is then

I = I₀ {1+ (1/2) V (Delta L₁-Delta L₂) cosz} (16)

with z = 2 * pi * f₀ * (1 / c) (Delta L₁-Delta L₂).

The tandem two-mode fiber interferometer behaves as a single interferometer with the difference in the delta L = Delta L ₁ -Delta L ₂ , with the difference that the interference contrast is reduced by a factor of 2. The intensity I according to equation (16) is shown schematically in FIGS. 3A and 3B. FIG. 3A shows the immediate area around delta L ₁ -Delta L ₂ = 0 (order p = 0; | Delta L ₁ -Delta L ₂ | some 100 µm). Fig. 3B illustrates a larger section is out of the range of values of Delta L ₁ L ₂ with -Delta "interference packages" height rer order (| p | 0).

For comparison, here are some numerical values for the tandem two-mode fiber interferometer the Michelson Interferometer compared:

Michelson interferometer (in air)

The full width at half maximum of the peaks in the function V (Delta L) is in the air for many commercial multi-mode laser diodes in the order of magnitude of 200 μm. The optical length l _{cav of} the laser resonator is approximately 1 mm. The distance between two maxima is about 2 mm. The height K of the maxima usually falls within the first 10 orders (| p | 10) to values of a few percent.

Tandem two-mode fiber interferometer

The difference delta n _eff = n (LP ₀₁ ) -n (LP ₁₁ ) of the effective refractive indices is strongly dependent on the fiber type. For a two-mode fiber for a lambda wavelength of 780 nm, it can lie roughly in the range between 0.001 and 0.015.

For a fiber with z. B. Delta n _eff = 0.01, the following parameters result: The half-width of the peaks of the function V (l) corresponds to a fiber length difference of the order of magnitude of (200 μm) / delta n _eff = 2 cm. So that the total path difference Delta L ₁ -Delta L _{2 of} the tandem interferometer is still safely within the "interference packet" of the 0th order (p = 0), the lengths of the two two-mode fibers 4 a and 4 b should accordingly match exactly a few millimeters. In practice, such tolerances can easily be kept.

The fiber length difference between two maxima of V (l) is (2 mm) / delta n _eff = 20 cm. The maximum 10th order occurs with a fiber length of 2 m. Due to the relatively large distance between two maxima of V (l), the fiber lengths can easily be adjusted so that the conditions according to equations (14) and (15) are met.

A preferred embodiment for the construction of a sensor for electrical field and voltage measurement according to the invention is shown in FIG. 5. In an electric alternating field E, a piezoelectric sensor element 6 (e.g. a quartz) modulates the length of the first two-mode fiber 4 a and thus also the phase difference between the two excited modes. The phase difference between the interfering waves at the output end of the second two-mode fiber 4 b is given by

Φ (t) = A * sin (omega * t) + Φ₁ ^O (t) - Φ₂ ^O (t). (17)

Φ₁ ^O (t) and Φ₂ ^O (t) are the phase differences between the modes of the two two-mode fibers 4 a and 4 b when there is no modulation. Φ₁ ^O (t) and Φ₂ ^O (t) can z. B. change due to temperature fluctuations (temperature-dependent fiber lengths). However, the temperature fluctuations of the path differences are very much smaller than the width of the peaks of the function V (Delta L), so that the interference contrast is practically not changed. Equation (16) is then

I = I₀ {1+ (1/2) Vcos (A * sin (omega * t) + Φ₁ ^O (t) -Φ₂ ^O (t))} (18)

The intensity I is shown in Fig. 4 for V = l as a function of the phase Φ. The homodyne detection method with active phase compensation described earlier can then be used for signal detection (see EP-A1-04 33 824 cited at the beginning). The difference Φ₁ ^O (t) -Φ₂ ^O (t) is regulated to pi / 2 modulo 2 * pi, the term A * sin (omega * t) is compensated by an additional phase-modulation. For this purpose, a piezoelectric modulator 7 is provided, which is controlled by a quadrature controller 10 . The quadrature controller 10 receives its input signal from two at the output end of the second two-mode fiber 4 b arranged detectors 8 a and 8 b, which convert the optical interference signal into electrical signals, from which the difference is formed by means of a downstream subtractor 9 . From the control signal of the quadrature controller 10 , the measurement signal is extracted by means of a high pass 11 and output 12 is made available at a signal.

In contrast to the known methods, the modulator does not modulate the sensor fiber itself, but the second two-mode fiber 4 b. The first two-mode fiber 4 a (the actual sensor fiber) can then remain limited to the immediate area of the sensor element 6 and does not have to bridge the distance between the sensor element and evaluation unit 13 . The first two-mode fiber bil det with the sensor element 6 together a compact and simply constructed sensor head 14 , which is only connected by insensitive monomode fibers 2 a, 2 b to the evaluation unit 13 , in which the second two-mode -Fiber 4 b and the modulator 7 including control are located. The terms A * sin (omega * t) and Φ₁ ^O (t) -Φ₂ ^O (t) can also - as already described earlier - be compensated with two separate modulators instead of the modulator 7 from FIG. 5.

The two (polarization-maintaining) two-mode fibers preferably have a core with an elliptical cross section. The fiber parameters can then be chosen so that only the LP ₀₁ - and the straight LP ₁₁ mode can be propagated for the wavelength of the laser diode (see the article by BY Kim et al. Cited at the beginning). In fibers with a round core, the odd LP ₁₁ mode would be able to spread at the same time as the even one.

The two monomode fibers 2 a and 2 b are preferably designed to maintain polarization. However, ordinary single-mode fibers can also be used. The polarization would then have to be actively controlled, as described in T. Pikaar et al., J. Lightw. Tech. 7, 1982-1887 (1989).

Overall, the invention results in a more robust, ge accurate and simply constructed sensor.

Reference list

1 light source
2 a, b monomode fiber
3 a, b, c splice
4 a, b two-mode fiber
5 detection unit
6 sensor element
7 modulator
8 a, b detector
9 subtractors
10 quadrature controls
11 high pass
12 signal output
13 evaluation unit
14 sensor head
E electric field
V interference contrast
I, I₀ intensity
Delta L path difference
Delta L₁ path difference
Delta L₂ path difference
LP₀₁ fiber fashion
LP₁₁ fiber fashion
l _cav resonator length
Φ phase

Claims (7)

1. Fiber-optic sensor for alternating electrical fields and voltages, comprehensive

• a) a light source ( 1 );
• b) a piezoelectric sensor element ( 6 );
• c) a first two-mode fiber ( 4 a) with an input end and an output end in which the LP ₀₁ basic mode and the straight LP ₁₁ mode can spread, and which at least partially on the sensor element ( 6 ) is fixed that a dimensional change of the sensor element ( 6 ) in an electric field leads to a change in length in the fiber; and
• d) means for measuring the field-related change in length of the fiber;

characterized in that

• e) the light source ( 1 ) is a multi-mode laser diode;
• f) the measuring means comprise a second, two-mode fiber ( 4 b) of the same type;
• g) the parameters of the two two-mode fibers ( 4 a, 4 b) are chosen so that the interference contrast (V) for the path differences (delta L ₁ , delta L ₂ ) of both modes in the individual two-mode fibers ( 4 a, 4 b) and for the sum of the path differences (Delta L ₁ ) + (Delta L ₂ ) each is approximately equal to zero; and
• h) between the light source ( 1 ) and the input end of the first two-mode fiber ( 4 a), and the output end of the first two-mode fiber ( 4 a) and the input end of the second two-mode fiber ( 4 b) each a single-mode fiber ( 2 a or 2 b) are provided for the transmission of the light.

2. Fiber optic sensor according to claim 1, characterized in that

• a) the two-mode fibers ( 4 a, b) have a core with an elliptical cross-section; and
• b) the monomode fibers ( 2 a, b) are polarization-maintaining.

3. Fiber optic sensor according to claim 2, characterized in that the two single-mode fibers ( 2 a, b) with the two two-mode fibers ( 4 a, b) are connected so that in the two two-mode Fibers ( 4 a, b), the two from spreadable modes by the light from the light source ( 1 ) are each excited with approximately the same intensity. 4. Fiber optic sensor according to claim 3, characterized in that the effective refractive index differences n (LP ₀₁ ) -n (LP ₁₁ ) for the two modes in the two two-mode fibers ( 4 a, 4 b) and Lengths of the two two-mode fibers ( 4 a, 4 b) are each the same. 5. Fiber optic sensor according to claim 4, characterized in that

• a) the second two-mode fiber ( 4 b) is at least partially fixed to a piezoelectric modulator ( 7 );
• b) two detectors ( 8 a, b) for measuring the intensities of the two modes are arranged at the output end of the second two-mode fiber ( 4 b);
• c) the output signals of the two detectors ( 8 a, b) reach the input of a quadrature controller ( 10 ) via a subtractor ( 9 );
• d) the output of the quadrature controller ( 10 ) controls the modulator ( 7 ); and
• e) the output signal of the quadrature controller ( 10 ) via a high-pass filter ( 11 ) on a signal output ( 12 ) will give ge.

6. Fiber optic sensor according to claim 5, characterized in that

• a) the light source ( 1 ), the second two-mode fiber ( 4 b) with the modulator ( 7 ) and the detectors ( 8 a, b), and the associated electronics ( 9 , 10 , 11 ) to form an evaluation unit ( 13 ) are summarized;
• b) the first two-mode fiber ( 4 a) with the sensor element ( 6 ) are arranged in a separate sensor head ( 14 ); and
• c) Sensor head ( 14 ) and evaluation unit ( 13 ) are connected to each other only by the two monomode fibers ( 2 a, b).